The Donation King title can be acquired in any of the major cities that Dalair appears in. Speak with Dalair to see the top 5 donors, but note that the #1 donor's donation amount will not appear. Acquire the quest from Dalair, then speak with the Donation Box beside him and donate as many mesos as you think will be required to beat everyone else. If you acquire a significantly larger amount of mesos than you think will be required to achieve the #1 rank, you will probably be able to hold the title for a longer amount of time.